The present invention relates to reducing electromagnetic interference (EMI) in electronic systems, and more particularly, to reducing secondary EMI emitted from a heat sink.
Electronic systems include current-carrying paths. For example, integrated circuits, such as processors used in personal computers and other electronic products, include current-carrying paths that connect electronic components, such as resistors, capacitors, and transistors, together. In addition, the substrates and circuit boards on which integrated circuits are packaged include current-carrying paths. As currents are switched on and off along current-carrying paths, electromagnetic radiation, which is sometimes referred to as electromagnetic interference (EMI), is radiated from these paths. Current-carrying paths that emit EMI are often referred to as primary EMI sources. EMI may also be re-radiated from a conductor that absorbs electromagnetic radiation. For example, an ungrounded heat sink that is thermally coupled to a processor can absorb electromagnetic radiation from the processor and re-radiate the absorbed radiation. Sources of re-radiated EMI are often referred to as secondary EMI sources.
EMI generated by either primary or secondary sources can interfere with electronic signals from other devices, such as those in communication systems. In the United States, the Federal Communications Commission (FCC) sets limits on EMI emitted by electronic devices. In other countries, agencies similar to the FCC also set limits on EMI emissions. To avoid exceeding the EMI emission limits set by government agencies, the manufacturers of electronic devices have developed shielding and grounding schemes to reduce the EMI emitted by electronic devices.
One shielding scheme used to reduce EMI in electronic systems requires completely surrounding the electronic system with a conductive shield connected to a ground. The shield is typically fabricated in as a single component having fixed mounting locations and which absorbs EMI produced by the electronic system. The absorbed EMI causes electrical charge to accumulate on the shield, and the ground connection provides a discharge path for the accumulated charge. Unfortunately, a shield designed as a single component having fixed mounting locations often interferes with the layout and packaging of other components in a system.
One grounding scheme for reducing secondary EMI that does not interfere with the layout and packaging of components in a system including a processor and a heat sink has two manufacturing steps. First, one end of a ground wire is attached to the heat sink. Second, the other end of the ground wire is attached to a ground connection. The ground wire is typically attached to the heat sink and ground connection using screws. This grounding scheme provides a reliable method of reducing some EMI, but the scheme has two disadvantages. First, the installation of the ground wire is labor intensive and therefore very expensive. Second, the ground wire is only effective at reducing EMI at low frequencies. For modem processors operating at high frequencies, this grounding scheme is ineffective.
For these and other reasons there is a need for the present invention.